Adding basic example to README.

Author: aentinger

README.md

@@ -23,5 +23,35 @@ This library works for
 
 ### Example
 ```C++
+#include <107-Arduino-CriticalSection.h>
 
+volatile int button_evt_counter = 0;
+
+void setup() {
+  Serial.begin(9600);
+  while (!Serial) { }
+
+  attachInterrupt(0, onButton_1_Pressed, RISING);
+  attachInterrupt(1, onButton_2_Pressed, RISING);
+}
+
+void loop() {
+  int copy_button_evt_counter;
+  {
+    /* Prevent change of button_evt_counter during readout. */
+    CriticalSection crit_sec;
+    copy_button_evt_counter = button_evt_counter;
+  }
+  Serial.println(copy_button_evt_counter);
+}
+
+void onButton_1_Pressed() {
+  CriticalSection crit_sec;
+  button_evt_counter++;
+}
+
+void onButton_2_Pressed() {
+  CriticalSection crit_sec;
+  button_evt_counter++;
+}
 ```